Enhanced Solar Hydrogen Evolution over In Situ GoldâPlatinum

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Enhanced solar hydrogen evolution over in situ gold- platinum bimetallic nanoparticle loaded Ti3+ self- doped titania photocatalysts RAHUL T K, Minu Mohan, and Neelakandapillai Sandhyarani ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02898 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced solar hydrogen evolution over in situ gold- platinum bimetallic nanoparticle loaded Ti3+ self- doped titania photocatalysts Theyyathum Kavil Rahul†, Minu Mohan‡ and Neelakandapillai Sandhyarani†*

†

Nanoscience Research Laboratory, School of Nano Science and Technology, National Institute of Technology Calicut, Calicut, Kerala, India, Fax: 91 495 2287250, Tel.: 91 495 2286537.

‡

School of Physics, Indian Institute of Science Education and Research (IISER TVM), Vithura, Thiruvananthapuram, Kerala 695551, India

*Corresponding author, [email protected] KEYWORDS. Photocatalytic water splitting; TiO2 inverse opal; Ti3+ self- doping; Photonic band gap; Slow photon effect; Solar light driven hydrogen evolution

ABSTRACT. This work presents a photochemical and thermal treatment strategy to prepare in situ gold- platinum bimetallic nanoparticle loaded titania photocatalysts with self-doped Ti3+ states (Au-Pt/Ti3+ nc-TiO2). In situ loading of Au-Pt bimetallic nanoparticles and Ti3+ selfdoping in TiO2 crystal lattice result in excellent solar light photocatalytic activity. The AuPt/Ti3+nc-TiO2 displays improved hydrogen evolution rate (98.53 mmol h-1 g-1) when compared to in situ gold loaded and in situ platinum loaded titania (Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2,

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respectively) photocatalysts. Au-Pt/Ti3+ nc-TiO2 photocatalyst is further restructured into titania inverse opal (Au-Pt/Ti3+ io-TiO2) photocatalyst by a facile colloidal photonic crystal (CPC) infiltration method. The Au-Pt/Ti3+ io-TiO2 photocatalyst displays superior solar hydrogen evolution profile (181.77 mmol h-1 g-1) compared to all the other photocatalysts investigated for hydrogen production experiment which makes them potential candidates for solar water splitting.

INTRODUCTION Hydrogen fuel generation by solar water splitting provides a clean and sustainable route to hydrogen economy. Hydrogen is considered as the ideal fuel for the future due to the depletion of fossil fuel resources as well as the environmental concerns arising from the usage of them.1–4 Photocatalytic hydrogen prodcution by TiO2 has received extensive research interest since the discovery of water photolysis by Fujishima and Honda in 1972.2,5,6 However visible/solar light driven hydrogen production efficiency of of TiO2 photocatalyst was found to be very low due to its wide bandgap (ca. 3.2 eV) lying in the ultra-violet range.7–9 Hence several modifications including non-metal doping, defect-generation, noble-metal deposition and physical restructuring have been implemented over the years to improve the efficiency of hydrogen production by TiO2 under visible/solar irradiation.10–14 Self-doping with Ti3+ has proved to be effective to reduce the wide badgap of TiO2 without the generation of any carrier recombination sites from doping agents.15,16 The presence of Ti3+ states can also improve the electron transfer inside TiO2 crystal lattice and enhance the efficiency of photocatalysis.17–21 The use of macroporous inverse opal based titania photocatalysts with slow phtoton effects has also been applied recently to improve photochemical hydrogen evolution.22,23 The approach described in this communication combines both chemical and morphological modifications by the restructuring of Ti3+ self-doped TiO2


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photocatalyst into inverse opaline materials. To date, the research related to the application of Ti3+ self-doped titania inverse opals for photocatalytic hydrogen fuel generation is very limited. Photochemical hydrogen evolution by TiO2 typically involves the use of noble metal cocatalysts such as Pt, Au, Ag, etc. which forms Schottky junctions at the metal-semiconductor interface.24–26 Among these metals Pt has been widely used as co-catalyst taking advantage of its excellent catalytic performance; however the poor visible light absorption limits its broad utilization under solar wavelength range.25 Recently, Au, Ag and Cu nanoparticles have been found to be effective for solar light driven hydrogen production as it can enhance visible light absorption due to localized surface plasmon resonance.27–30 However the solar efficiency and hence the hydrogen yield is often compromised, which limits the practical applications of these photocatalyst systems. Latterly, the use of bimetallic co-catalysts such as alloys and core-shell nanostructures have offered superior activities for photocatalytic hydrogen production.31,32 In this work, a novel strategy based on photochemical followed by thermal treatment was implemented to prepare in situ Au-Pt bimetallic nanoparticle loaded Ti3+ self-doped inverse opaline TiO2 photocatalysts with excellent solar hydrogen yied of the order of several sub-moles. The method developed in this work involves the use of titanium (iv) butoxide as precursor for titania and photochemical reduction of Pt and Au for the in situ loading on titania followed by a rapid thermal treatment which is found to be beneficial for the formation of Ti3+ self-doped TiO2. To the best of our knowledge, this is the first report to prepare in situ Au-Pt bimetallic nanoparticle loaded Ti3+ self-doped titania inverse opal photocatalyst towards solar hydrogen production. The role of precursor on the chemical modification of titania was also investigated. The use of titanium butoxide instead of the commonly used titanium isopropoxide resulted in the formation of Ti3+ self-doped TiO2. The Au-Pt/Ti3+ nc-TiO2 photocatalysts displayed excellent hydrogen


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evolution profile under simulated solar irradiation. The effect of co-catalyst on hydrogen yield was also investigated by preparing both Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2 photocatalysts. In situ Au-Pt/ Ti3+ io-TiO2 photocatalyst with slow photon effects tuned to match with the electronic bandgap absorption region of titania displayed superior solar hydrogen production profile compared to recent reports related to inverse opal assisted hydrogen production experiments.22,23,33–35 EXPERIMENTAL SECTION Materials Titanium (IV) butoxide (97%, Sigma-Aldrich), titanium (IV) isopropoxide (97%, SigmaAldrich), tetrachloroauric acid (49% metal basis, Spectrochem), chloroplatinic acid hexahydrate (37.5% metal basis, Sigma-Aldrich), trifluoroacetic acid (99%, Sigma-Aldrich), hydrochloric acid (35%, Merck) and ethanol (99.9%, Merck) were purchased from the indicated suppliers and used without doing additional purification. Monodisperse polystyrene (PS) latex microspheres (of size 215 nm, 2.5 wt% suspensions in water) were purchased from Alfa Aesar. All the experiments were carried out by using ultra pure water obtained from Millipore source. Preparation of photocatalyst samples PS CPC templates were prepared by a vertical self-assembly method as per the procedure described in our previous report.10 The modified TiO2 precursor was prepared as follows: 1 mL of Titanium (IV) butoxide, 0.8 mL of trifluoroacetic acid and 0.2 mL of HCl were mixed under sonication for 45 minutes. Then 500 µL of 0.02M tetrachloroauric acid or 0.02 M chloroplatinic acid hexahydrate or 250 µL each of both in ethanol was added to the above mixture and


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sonicated for 15 minutes. The TiO2 precursor containing both tetrachloroauric acid and chloroplatinic acid hexahydrate solutions were exposed to 190 W Hg/Xe lamp for nearly one hour to bring about the photochemical reduction of gold and platinum nanoparticles. Then the modified precursor was carefully dropped over the PS CPC templates to infiltrate the interstitial spaces of the CPC films. The samples were kept in hot-air oven at 80°C overnight for drying. During calcination, all the samples were rapidly heated to 400°C at a heating rate of ca. 15°C/ min. The calcination was continued at 400°C for 4 hours to form the inverse opaline and nanocrystalline powder samples. The in situ gold, platinum monometallic and gold-platinum bimetallic nanoparticle loaded nanocrystalline titania samples prepared using titanium butoxide precursor are referred as Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2, Au-Pt/Ti3+ nc-TiO2, respectively. The inverse opaline samples were denoted as Au/Ti3+ io-TiO2, Pt/ Ti3+ io-TiO2 and Au-Pt/ Ti3+ io-TiO2. The inverse opaline samples were scraped off from the glass substrates for the hydrogen production experiments. In situ monometallic Au and bimetallic Au-Pt loaded nanocrystalline titania powder samples were prepared by replacing titanium butoxide with titanium isopropoxide under the similar conditions and were denoted as Au/nc-TiO2 and Au-Pt/nc-TiO2, respectively. In situ bimetallic Au-Pt loaded titania inverse opal sample was denoted as Au-Pt/io-TiO2. Characterization Techniques The morphologies of the samples were analyzed using a Hitachi SU6600 variable pressure field emission scanning electron microscope. Energy dispersive spectroscopy and elemental mapping were performed with a Horiba EMAX 137eV EDS unit attached with the same microscope. Transmission electron microscopy (TEM) images of the samples were captured using a JEOL JEM -2100 high resolution transmission electron microscope. Elemental composition was investigated using an Axis Ultra Shimadzu X-ray photoelectron spectrometer (XPS). The X-ray


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diffraction patterns (XRD) were obtained on a Rigaku Smartlab X-ray diffractometer using CuKα as the irradiation source. Gold and platinum contents of the samples were estimated by a Perkin Elmer Optima 5300 DV inductively coupled plasma atomic emission spectrometer (ICP). Raman spectra were recorded with a Witec Alpha300 RA Raman microscope. Electron paramagnetic resonance analysis was carried out (at room temperature/X-band, in the range of 0500 mT) with a JEOL Model JES FA200 EPR spectrometer. The diffuse absorbance spectra of the powder samples were obtained on a UV 2600 Schimadzu Spectrophotometer with BaSO4 as the reflectance standard. Reflectance spectra of the inverse opal samples at normal incidence were collected using a UV/VIS/NIR FLAME-T-XR1-ES Ocean Optics Fiber Optic Spectrometer. Photoelectrochemical measurements were performed with a CHI 400A electrochemical workstation with a three electrode system and a quartz cell filled with 0.1 M Na2SO4 aqueous solution. The photocatalyst samples were suspended in 0.02% nafion solution (in ethanol) and were coated on indium-tin-oxide (ITO) glass substrates. Thin films of the photocatalyst samples (with area ~1cm2) over ITO glass substrates were used as the working electrodes. A platinum wire and an Ag/AgCl electrode were employed as the counter and reference electrodes, respectively. Photocurrent response of the photocatalysts were measured at a potential bias of 0.3 V, under the irradiation of 200 W Hg/Xe lamp (working at 190 W) with TS short pass filter-400 nm and Newport AM 1.5 G filter. The incident photon-to-electron conversion efficiency (IPCE) measurements were carried at different wavelengths at a potential bias of 1 V (Vs. Ag/AgCl electrode and Pt-mesh electrode) in 1M KOH solution. A 250 W xenon lamp coupled to a Newport monochromator was used as the light source and light was chopped at 40 Hz using a chopper blade. A computer controlled SR830 OSP model lock-in amplifier (Stanford Research Systems Inc.) was used to measure the current response, and a


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NIST calibrated silicon photodiode was used to find the spectral power distribution of the incident light. Photocatalytic experiment Photocatalytic water splitting experiments were conducted in an in-house fabricated quartz reaction cell of 60 mL capacity connected to glass-enclosed gas circulation system, provided with a port for collecting the H2 gas at regular intervals of time. In a typical run, 20 mg of the photocatalyst sample was suspended in 20 mL 10% ethanol aqueous solution in the quartz vessel. The nanocrystalline powder samples were sonicated for 15 minutes prior to the reaction to break aggregates if any. After evacuating the system for half an hour, the quartz reaction cell containing the reaction mixture was exposed to simulated solar light to execute the photocatalytic reaction maintaining the temperature at 35°C. Solar light illumination was obtained by a to 200 W Hg/Xe lamp (working at 190 W) equipped with an AM 1.5 G filter (Newport Corporation) by adjusting the light intensity to ca. 100 mW/cm2 or 150 mW/cm2 and wavelength range to 350 nm to 700 nm. Figure S1 shows the spectral output of 190 W Hg/Xe lamp with and without AM 1.5 filter. Hydrogen production experiments under UV irradiation (400 nm). Figure 7 shows the hydrogen production profile and the rates of hydrogen evolution for the photocatalysts under UV irradiation. Pt/Ti3+ nc-TiO2 displayed enhanced hydrogen production profile when compared to Au/Ti3+ nc-TiO2 and Au-Pt/Ti3+ nc-TiO2. The hydrogen production rate observed for Pt/Ti3+ nc-TiO2 photocatalyst was 143.39 mmol h-1 g-1, whereas Au/ Ti3+ nc-TiO2 and AuPt/Ti3+ nc-TiO2 displayed hydrogen production rates of 49.96 mmol h-1 g-1 and 82.55 mmol h-1 g1

, repectively. This proved that Pt is a better catalyst for H2 generation under UV irradiation.

However Au-Pt/Ti3+io-TiO2 photocatalyst displayed the highest hydrogen evolution rate (232.22 mmol h-1 g-1) when compared to the other photocatalysts under UV irradiation. This evidence the enhanced light absorption by the Au-Pt/Ti3+io-TiO2 due to the photonic effects within this wavelength range (>400 nm).

Figure 7. (A) Hydrogen production profile and (B) the rate of hydrogen evolution for Au/ Ti3+ nc-TiO2, Pt/Ti3+ nc-TiO2, Au-Pt/Ti3+ nc-TiO2 and Au-Pt/Ti3+io-TiO2 photocatalysts under UV


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irradiation with visible cut-off filter (>400 nm). The experiments were done by adjusting the light intensity to ca. 100 mW/cm2. Superior solar hydrogen profiles observed for the photocatalysts prepared from titanium butoxide precursor is attributed to the presence of Ti3+ self-doping and oxygen vacancies in the TiO2 crystal lattice. Higher rate of hydrogen evolution was obtained for Au-Pt/Ti3+ nc-TiO2 photocatalysts than that of Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2 photocatalysts under simulated solar irradiation. The increase in H2 evolution rate from Pt to Au-containing Ti3+ nc-TiO2 indicates that gold contributes predominantly to the light absorption and hence activity under simulated solar light irradiation. Indeed, Pt is a better catalyst for H2 generation under UV irradiation but shows less activity than Au under simulated solar light conditions. It is evident from the UV-vis DRS results that the catalyst showed LSPR characteristics due to the in situ loading of Au nanoparticles. Surface plasmon resonance of gold nanoparticles lead to enhanced absorption of visible light and hence more activity in hydrogen production. Fermi level equilibration can occur in this catalyst and hot electron transfer from gold to titania is possible from the plasmon state to the conduction band of the semiconductor.56,57,58 This improves the electron density in the conduction band of TiO2 which facilitate the improved rate of photochemical reaction. It was also observed that Au-Pt bimetallic nanoparticles loaded photocatalysts displayed enhanced activity than individual Au and Pt monometallic nanoparticle loaded photocatalysts. In addition to the contribution from Au nanoparticles; Pt nanoparticles and Au-Pt bimetallic nanoparticles will also contribute to hydrogen production reaction. The higher rate of hydrogen evolution of Au-Pt/Ti3+ nc-TiO2 photocatalysts than that of Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2 photocatalysts indicates that Au-Pt bimetallic nanoparticles are good cocatalysts. The enhanced electronic charge transfer effects between neghbouring atoms of


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different elements could be the possible reason for the improved activity of the bimetallic nanoparticles.59 An enhancement in solar hydrogen evolution was demonstrated for Au-Pt/Ti3+ io-TiO2 photocatalyst compared to Au-Pt/Ti3+ nc-TiO2 photocatalyst. This can be explained based on the synergism between the chemical effects (Ti3+ self-doping) and the physical effects (photonic effect) materializing over Au-Pt/Ti3+ TiO2 inverse opal photocatalyst. Recently we have reported a five-fold enhancement in hydrogen evolution with in situ gold loaded fluorinated titania inverse opal photocatalyst relative to nanocrystalline titania photocatalyst due to the electronic band gap absorption of TiO2 within the photonic bandgap.35 Slow photon effects at both blueedge and red-edge of the photonic bandgap were found to be effective for enhancing photocatalysis.60–62 Figure 8 shows the schematic illustration of the mechanism of solar light driven hydrogen evolution over Au-Pt/Ti3+ io-TiO2 photocatalyst. The physical restructuring of the nanocrystalline titania into inverse opal resulted in better solar light harvesting due to the absorption enhancement by Bragg scattering and slow photon effect (named as photonic effect) inside the photonic bandgap.61,63 The increased effective path length of light due to this photonic effect resulted in enhanced light absorption by Au-Pt/Ti3+ io-TiO2 segments since its electronic absorption edge (~416 nm) was inside the frequency edges of the PBG (~350 nm and ~465 nm). This enhanced light absorption causes excitation of more electrons from the valence band to the conduction band of titania. Since we used Au-Pt/Ti3+ io-TiO2 in the form of inverse opal segments in aqueous medium, incident angle may not be unique under light irradiation as they continuously rotate in the suspension exposing different facets. Hence the inverse opals can act as dielectric mirrors which cause strong light localization inside the structure. In addition to this, diffuse scattering and multiple internal scattering can also contribute to the enhanced the optical


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absorption of titania inverse opal segments. Additionally, more electrons could be trapped by the intermediate energy level induced by Ti3+ self-doping. These electrons will be subsequently transferred onto in situ loaded Au-Pt bimetallic co-catalysts where water reduction takes place resulting in hydrogen evolution. Besides Pt monometallic nanoparticles can act as the co-catlysts and may also contribute to hydrogen evolution under simulated solar light irradiation. The hot electron injection57,58 from the Au monometallic nanoparticles into titania will also improve the electron density in the conduction band of the photocatalyst as discussed earlier. The photocatalyst samples without any bimetallic nanoparticle loading or Ti3+ self doping or photonic effects show lower solar light activity due to the lack of synergistic effect between chemical and physical enhancements.


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Figure 8. Schematic of the proposed mechanism of hydrogen evolution over Au-Pt/Ti3+ io-TiO2 photocatalyst under solar light illumination. CONCLUSION In summary, in situ gold-platinum bimetallic nanoparticle loaded titania photocatalysts with self-doped Ti3+ states were prepared based on a novel synthetic strategy. In situ formation of Au-Pt bimetallic nanoparticles and Ti3+ self-doping in TiO2 resulted in excellent solar hydrogen production under simulated solar irradiation. The solar hydrogen profile of the photocatalyst was further enhanced by adopting inverse opal structure with photonic effects matching the electronic absorption region of titania. We hope this work would provide a motif for the developement of highly solar light acive titania photocataysts by the combination of chemical and physical modifications.


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ASSOCIATED CONTENT Supporting Information. The photograph and video of the hydrogen bubble formation over AuPt/Ti3+ io-TiO2 photocatalyst, the photograph of the experimental set-up used for recording the hydrogen generation, source spectral output, hydrogen production profile and the rates of hydrogen production for Au/Ti3+ io-TiO2 and Pt/Ti3+ io-TiO2 photocatalysts under simulated sunlight irradiation, the hydrogen production profile as well as the rate of hydrogen evolution for Au/nc-TiO2, Au-Pt/nc-TiO2 and Au-Pt/io-TiO2 photocatalysts prepared by titanium isopropoxide as a precursor for titania, stability test of the catalyst, Comparison of literature and present work, TEM images of Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2 samples, XRD patterns and Raman spectra for Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2, The XPS survey spectrum and the XPS spectra of F 1s, C 1s, Au4f and Pt4f region for Au-Pt/Ti3+ nc-TiO2 sample, EPR spectra for Au-Pt/Ti3+ nc-TiO2 and Au-Pt/Ti3+ io-TiO2 samples after hydrogen reaction under simulated solar light, XPS of Ti 2p region for Au-Pt/Ti3+ nc-TiO2 samples after hydrogen production reaction, The diffuse absorbance spectra of Au/Ti3+ nc-TiO2 and Pt/Ti3+ nc-TiO2 samples, and Raman spectra obtained for Au-Pt/nc-TiO2 sample. AUTHOR INFORMATION Corresponding Author * Dr. N. Sandhyarani. E-mail: [email protected]


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ACKNOWLEDGMENT The authors thank Shiny Joseph and Vibin. A.P. at the Chemical Engineering Department of NIT Calicut for providing the gas chromatograph facility. We express our gratitude to N.K. Divya and P.P. Pradyumnan at the Physics department of University of Calicut for their help with XRD analysis. Manoj A.G. Namboothiry at the Physics Department of IISER Trivandrum is acknowledged for extending the IPCE facility which was supported from Solar Energy Research Initiative, DST (Government of India). We are grateful to Vinay T.V. of School of Nano Science and Technology, NIT Calicut for his help with Celestron Handheld Digital Microscope. N. Sandhyarani is thankful to DST, DBT, CSIR and KSCSTE for the financial support to Nanoscience Research Laboratory. ABBREVIATIONS CPC, colloidal photonic crystal; PS, polystyrene; EBG, electronic bandgap; PBG, photonic bandgap Pt/Ti3+ nc-TiO2, platinum monometallic nanoparticle loaded Ti3+ self-doped nanocrystalline titania; Pt/Ti3+ io-TiO2, platinum monometallic nanoparticle loaded Ti3+ selfdoped inverse opal titania; Au/Ti3+ nc-TiO2, gold-monometallic nanoparticle loaded Ti3+ selfdoped nanocrystalline titania, Au/Ti3+ io-TiO2, gold-monometallic nanoparticle loaded Ti3+ selfdoped inverse opal titania, Au-Pt/Ti3+ nc-TiO2, gold-platinum bimetallic nanoparticle loaded Ti3+ self-doped nanocrystalline titania, Au-Pt/Ti3+ io-TiO2, gold-platinum bimetallic nanoparticle loaded Ti3+ self-doped titania inverse opal. REFERENCES (1)

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SYNOPSIS This work reports the preparation of in situ gold-platinum bimetallic nanoparticle loaded Ti3+ self-doped

titania

photocatalysts

with

excellent

solar

hydrogen

evolution

profile.

The state-of-the-art photocatalytic activity originates from the chemically modified and physically restructured titania photocatalysts would provide a sustainable route for producing hydrogen fuel under solar light .

Table of Contents artwork/ Abstract graphics


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Enhanced Solar Hydrogen Evolution over In Situ GoldâPlatinum

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